1. Field of the Invention
This invention relates to composite sheet material and more particularly, to a method of preparing a composite sheet material from a polyurethane polymer and porous sheet material.
2. Description of the Prior Art
Resin impregnated sheet materials such as cloth, batts, waterleaves and the like are well known in the art. These resin impregnated sheet materials are useful for a plurality of purposes including imitation leather in the form of vinyls and the like, structural sheet materials such as conveyor belts and similar products.
Prior art methods of impregnating a particular web involve the impregnation or coating of a porous material with a polymeric resin such as a polyurethane, vinyl or a similar material. Polyurethanes have met with wide acceptance as a coating or impregnating composition due to their capability of wide variation in chemical and physical properties, particularly their flexibility and chemical resistance. In impregnating the porous sheet material with a polymeric resin, several techniques have been employed. One such prior art method involves the use of the polymeric resin in an organic solvent system wherein the sheet material is dipped in the solution and the solvent is removed therefrom. These solvent systems are undesirable since the solvent, in many cases, is toxic and must either be recovered for reuse or discarded. These solvent systems are expensive and do not necessarily provide a desirable product since upon evaporation of the solvent from the impregnated porous sheet material, the resin tends to migrate to provide a non-homogeneous impregnation of the porous sheet material resulting in resin richness toward the surface of the sheet material rather than uniform impregnation.
In order to alleviate the problems with solvent systems, certain aqueous polymeric systems have been proposed. In forming impregnated sheet materials by impregnation with aqueous polymers the aqueous portion must be removed. Again heat is required and migration of the polymer to the surfaces of the impregnated sheet material is encountered.
In one method of combining polyurethane solutions with porous substrates the polymer is applied in an organic solvent to a substrate, such as a needle punched polyester batt. The polymer-substrate composite is subsequently bathed with a mixture of organic solvent for the polymer and a non-solvent for the polymer that is at least partially miscible with the solvent until the layer is coagulated into a cellular structure of interconnected micropores. The solvent is removed from the coating layer along with the non-solvent to produce a solvent free microporous layer. Although this process yields acceptable properties for a polyurethane impregnated fabric, it has the disadvantage of an organic solvent system particularly when high performance polyurethanes are utilized which require relatively toxic and high boiling solvents. An example of this method is disclosed in U.S. Pat. No. 3,208,875.
In another method polyurethane dispersions in organic vehicles have been proposed and used to coat porous substrates such as is disclosed in U.S. Pat. No. 3,100,721. In this system a solution is applied to a substrate and coagulated by further addition of a non-solvent for the polymer. Although this approach has been used with some success, it involves two major limitations: (1) the vehicle of the solution is substantially organic since relatively small amounts of non-solvent, preferably water, are needed to form a dispersion; and (2) there is a narrow useful range of added non-solvent so that reproducible results are difficult to obtain.
One particularly useful method of preparing composite sheet material by impregnating a porous substrate is disclosed in U.S. Pat. No. 4,171,391 incorporated herein by reference. In this system a porous sheet material is impregnated with an aqueous ionic dispersion of a polyurethane and the impregnant is coagulated therein. The composite is then dried to form a composite sheet material.
In one improvement over the method disclosed in U.S. Pat. No. 4,171,391, fibrous needled batts are fully impregnated with a polyurethane dispersion and coagulated by dipping the impregnant in an aqueous solution of acid such as acetic acid. This improved method is more fully disclosed in U.S. patent application Ser. Nos. 188,329 and 188,330, filed Sept. 18, 1980 by John McCartney, both incorporated herein by reference.
Although the prior art processes described in U.S. Pat. No. 4,171,391 and U.S. patent application Ser. Nos. 188,329 and 188,330 are successful in alleviating solvents in impregnating porous sheet materials with polyurethanes, a disadvantage in these processes arises in the coagulation step. The coagulation in these processes requires that the porous sheet material impregnated with polyurethane dispersion be contacted with an aqueous solution of a counterion. Thus, the impregnant must be washed free of the excess counterion prior to drying to form the composite sheet material. Further, these counterions can be supplied by low molecular weight organic acids such as acetic acid which can add an objectionable odor to the final product and also adds expense thereto. Further, in these prior art processes the coagulation rate is determined by the rate of diffusion of the coagulating agent, such as aqueous acid, into the saturated sheet material and subsequent diffusion of the products of coagulation in the washing step. In thicker felt products, such as 0.25 inch or greater, contact times as long as 30 minutes may be required to effect coagulation. Thus, it is desirable to reduce the time of coagulation.
In accordance with the present invention a process for forming a composite sheet material is provided which simplifies coagulation of polyurethane dispersions in porous sheet materials.
Further in accordance with the present invention, a polyurethane composition is provided which simplifies the coagulation of the polyurethane dispersion in the porous sheet material.